Abstract

The discovery of atomically thin layered materials such as graphene and transition metal dichalcogenides has unveiled the unique exploration of novel fundamental physics and device applications in two-dimensions. Characterization of their crystal symmetry and subsequent electronic properties are prominent to realize the full potential of these reduced dimensional systems, which fundamentally determine the topology, chirality and rich interfacial physics. Second harmonic generation (SHG), a nonlinear optical effect, is sensitive to crystal symmetry and electronic structures, which proves to be one of the most powerful yet simple technique to capture the essence physics. On the other hand, the 2D nature of layered materials enables large tunability in its physical properties with a number of external stimuli, which in turn paves the way for the development of 2D nonlinear optoelectronic applications. In this review, we overview recent efforts employing second harmonic generation spectroscopy and microscopy to probe lattice structures and dipole polarizations in two-dimensional transition metal dichalcogenide and polar materials. In addition, multiple external stimuli used to control SHG as potential optoelectronic devices are covered. We conclude with a perspective on the future directions of exploration on emerging 2D magnetic and topological materials based on SHG spectroscopy.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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2018 (4)

Y. Song, S. Hu, M. L. Lin, X. Gan, P. H. Tan, and J. Zhao, “Extraordinary Second Harmonic Generation in ReS 2 Atomic Crystals,” ACS Photonics 5(9), 3485–3491 (2018).
[Crossref]

J. Xiao, H. Zhu, Y. Wang, W. Feng, Y. Hu, A. Dasgupta, Y. Han, Y. Wang, D. A. Muller, L. W. Martin, P. Hu, and X. Zhang, “Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking,” Phys. Rev. Lett. 120, 227601 (2018).

X. T. Gan, C. Y. Zhao, S. Q. Hu, T. Wang, Y. Song, J. Li, Q. H. Zhao, W. Q. Jie, and J. L. Zhao, “Microwatts continuous-wave pumped second harmonic generation in few- and mono-layer GaSe,” Light Sci. Appl. 7, 17126 (2018).

Y. Song, R. Tian, J. Yang, R. Yin, J. Zhao, and X. Gan, “Second Harmonic Generation in Atomically Thin MoTe2,” Adv. Opt. Mater. 6(17), 1701334 (2018).
[Crossref]

2017 (4)

Y. Wang, J. Xiao, H. Zhu, Y. Li, Y. Alsaid, K. Y. Fong, Y. Zhou, S. Wang, W. Shi, Y. Wang, A. Zettl, E. J. Reed, and X. Zhang, “Structural phase transition in monolayer MoTe2 driven by electrostatic doping,” Nature 550(7677), 487 (2017).

L. Wu, S. Patankar, T. Morimoto, N. L. Nair, E. Thewalt, A. Little, J. G. Analytis, J. E. Moore, and J. Orenstein, “Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals,” Nat. Phys. 13(4), 350–355 (2017).
[Crossref]

A.-Y. Lu, H. Zhu, J. Xiao, C.-P. Chuu, Y. Han, M.-H. Chiu, C.-C. Cheng, C.-W. Yang, K.-H. Wei, Y. Yang, Y. Wang, D. Sokaras, D. Nordlund, P. Yang, D. A. Muller, M.-Y. Chou, X. Zhang, and L.-J. Li, “Janus monolayers of transition metal dichalcogenides,” Nat. Nanotechnol. 12, 744 (2017).

C. Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals,” Nature 546(7657), 265–269 (2017).
[Crossref] [PubMed]

2016 (8)

X. Zhang, Q. H. Tan, J. B. Wu, W. Shi, and P. H. Tan, “Review on the Raman spectroscopy of different types of layered materials,” Nanoscale 8(12), 6435–6450 (2016).
[Crossref] [PubMed]

K. S. Novoselov, A. Mishchenko, A. Carvalho, A. H. Castro Neto, and O. Road, “2D materials and van der Waals heterostructures,” Science 353(6298), aac9439 (2016).
[Crossref] [PubMed]

M. Zhao, Z. Ye, R. Suzuki, Y. Ye, H. Zhu, J. Xiao, Y. Wang, Y. Iwasa, and X. Zhang, “Atomically phase-matched second-harmonic generation in a 2D crystal,” Light Sci. Appl. 5(8), e16131 (2016).
[Crossref] [PubMed]

R. Beams, L. G. Cançado, S. Krylyuk, I. Kalish, B. Kalanyan, A. K. Singh, K. Choudhary, A. Bruma, P. M. Vora, F. Tavazza, A. V. Davydov, and S. J. Stranick, “Characterization of Few-Layer 1T′ MoTe2 by Polarization-Resolved Second Harmonic Generation and Raman Scattering,” ACS Nano 10(10), 9626–9636 (2016).
[Crossref] [PubMed]

M. Merano, “Nonlinear optical response of a two-dimensional atomic crystal,” Opt. Lett. 41(1), 187–190 (2016).
[Crossref] [PubMed]

S. Manz, M. Matsubara, T. Lottermoser, J. Büchi, A. Iyama, T. Kimura, D. Meier, and M. Fiebig, “Reversible optical switching of antiferromagnetism in TbMnO3,” Nat. Photonics 10(10), 653–656 (2016).
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D. Afanasiev, B. A. Ivanov, A. Kirilyuk, T. Rasing, R. V. Pisarev, and A. V. Kimel, “Control of the Ultrafast Photoinduced Magnetization across the Morin Transition in DyFeO3,” Phys. Rev. Lett. 116, 097401 (2016).

Y. Li, K.-A. N. Duerloo, K. Wauson, and E. J. Reed, “Structural semiconductor-to-semimetal phase transition in two-dimensional materials induced by electrostatic gating,” Nat. Commun. 7(1), 10671 (2016).
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2015 (10)

R. Trivedi, U. K. Khankhoje, and A. Majumdar, “Cavity-Enhanced Second-Order Nonlinear Photonic Logic Circuits,” Phys. Rev. Appl. 5(5), 54001 (2015).
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B. Q. Lv, H. M. Weng, B. B. Fu, X. P. Wang, H. Miao, J. Ma, P. Richard, X. C. Huang, L. X. Zhao, G. F. Chen, Z. Fang, X. Dai, T. Qian, and H. Ding, “Experimental Discovery of Weyl Semimetal TaAs,” Phys. Rev. X 5, 031013 (2015).

A. A. Soluyanov, D. Gresch, Z. Wang, Q. Wu, M. Troyer, X. Dai, and B. A. Bernevig, “Type-II Weyl semimetals,” Nature 527(7579), 495–498 (2015).
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E. A. Eliseev, S. V. Kalinin, and A. N. Morozovska, “Finite size effects in ferroelectric-semiconductor thin films under open-circuit electric boundary conditions,” J. Appl. Phys. 117(3), 034102 (2015).
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K. L. Seyler, J. R. Schaibley, P. Gong, P. Rivera, A. M. Jones, S. Wu, J. Yan, D. G. Mandrus, W. Yao, and X. Xu, “Electrical control of second-harmonic generation in a WSe2 monolayer transistor,” Nat. Nanotechnol. 10(5), 407–411 (2015).
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X. Wang, A. M. Jones, K. L. Seyler, V. Tran, Y. Jia, H. Zhao, H. Wang, L. Yang, X. Xu, and F. Xia, “Highly anisotropic and robust excitons in monolayer black phosphorus,” Nat. Nanotechnol. 10(6), 517–521 (2015).
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X. Wang and F. Xia, “Van der Waals heterostructures: Stacked 2D materials shed light,” Nat. Mater. 14(3), 264–265 (2015).
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H. Zhu, Y. Wang, J. Xiao, M. Liu, S. Xiong, Z. J. Wong, Z. Ye, Y. Ye, X. Yin, and X. Zhang, “Observation of piezoelectricity in free-standing monolayer MoS₂,” Nat. Nanotechnol. 10(2), 151–155 (2015).
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X. Duan, C. Wang, A. Pan, R. Yu, and X. Duan, “Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges,” Chem. Soc. Rev. 44(24), 8859–8876 (2015).
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2014 (12)

D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides,” ACS Nano 8(2), 1102–1120 (2014).
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Y. Li, A. Chernikov, X. Zhang, A. Rigosi, H. M. Hill, A. M. Van Der Zande, D. A. Chenet, E.-M. Shih, J. Hone, and T. F. Heinz, “Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe,” Phys. Rev. B 90, 205422 (2014).

K. F. Mak, K. L. McGill, J. Park, and P. L. McEuen, “Valleytronics. The valley Hall effect in MoS₂ transistors,” Science 344(6191), 1489–1492 (2014).
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W. Wu, L. Wang, Y. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T. F. Heinz, J. Hone, and Z. L. Wang, “Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics,” Nature 514(7523), 470–474 (2014).
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Z. Ye, T. Cao, K. O’Brien, H. Zhu, X. Yin, Y. Wang, S. G. Louie, and X. Zhang, “Probing excitonic dark states in single-layer tungsten disulphide,” Nature 513(7517), 214–218 (2014).
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S. Zhang, J. Yang, R. Xu, F. Wang, W. Li, M. Ghufran, Y. W. Zhang, Z. Yu, G. Zhang, Q. Qin, and Y. Lu, “Extraordinary photoluminescence and strong temperature/angle-dependent Raman responses in few-layer phosphorene,” ACS Nano 8(9), 9590–9596 (2014).
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K. He, N. Kumar, L. Zhao, Z. Wang, K. F. Mak, H. Zhao, and J. Shan, “Tightly Bound Excitons in Monolayer WSe2,” Phys. Rev. Lett. 113, 026803 (2014).

A. R. Klots, A. K. M. Newaz, B. Wang, D. Prasai, H. Krzyzanowska, J. Lin, D. Caudel, N. J. Ghimire, J. Yan, B. L. Ivanov, K. A. Velizhanin, A. Burger, D. G. Mandrus, N. H. Tolk, S. T. Pantelides, and K. I. Bolotin, “Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy,” Sci. Rep. 4, 6608 (2014).

M. M. Ugeda, A. J. Bradley, S. F. Shi, F. H. da Jornada, Y. Zhang, D. Y. Qiu, W. Ruan, S. K. Mo, Z. Hussain, Z. X. Shen, F. Wang, S. G. Louie, and M. F. Crommie, “Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor,” Nat. Mater. 13(12), 1091–1095 (2014).
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A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, O. B. Aslan, D. R. Reichman, M. S. Hybertsen, and T. F. Heinz, “Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2,” Phys. Rev. Lett. 113(7), 076802 (2014).
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X. Yin, Z. Ye, D. A. Chenet, Y. Ye, K. O’Brien, J. C. Hone, and X. Zhang, “Edge nonlinear optics on a MoS₂ atomic monolayer,” Science 344(6183), 488–490 (2014).
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2013 (10)

H. Yuan, M. S. Bahramy, K. Morimoto, S. Wu, K. Nomura, B. J. Yang, H. Shimotani, R. Suzuki, M. Toh, C. Kloc, X. Xu, R. Arita, N. Nagaosa, and Y. Iwasa, “Zeeman-type spin splitting controlled by an electric field,” Nat. Phys. 9(9), 563–569 (2013).
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S. Wu, J. S. Ross, G. Bin Liu, G. Aivazian, A. Jones, Z. Fei, W. Zhu, D. Xiao, W. Yao, D. Cobden, and X. Xu, “Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2,” Nat. Phys. 9(3), 149–153 (2013).
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L. Kou, T. Frauenheim, and C. Chen, “Nanoscale Multilayer Transition-Metal Dichalcogenide Heterostructures: Band Gap Modulation by Interfacial Strain and Spontaneous Polarization,” J. Phys. Chem. Lett. 4(10), 1730–1736 (2013).
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S. Hong, J. I. Dadap, N. Petrone, P. Yeh, J. Hone, and R. M. Osgood, “Optical Third-Harmonic Generation in Graphene,” Phys. Rev. X 3(2), 021014 (2013).
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A. K. Geim and I. V. Grigorieva, “Van der Waals heterostructures,” Nature 499(7459), 419–425 (2013).
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K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, “Tightly bound trions in monolayer MoS2,” Nat. Mater. 12, 207 (2013).

Y. Li, Y. Rao, K. F. Mak, Y. You, S. Wang, C. R. Dean, and T. F. Heinz, “Probing symmetry properties of few-layer MoS2 and h-BN by optical second-harmonic generation,” Nano Lett. 13(7), 3329–3333 (2013).
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W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P.-H. Tan, and G. Eda, “Evolution of electronic structure in atomically thin sheets of WS2 and WSe2.,” ACS Nano 7(1), 791–797 (2013).
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S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, and J. E. Goldberger, “Progress, challenges, and opportunities in two-dimensional materials beyond graphene,” ACS Nano 7(4), 2898–2926 (2013).
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2012 (6)

Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nat. Nanotechnol. 7(11), 699–712 (2012).
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T. Cao, G. Wang, W. Han, H. Ye, C. Zhu, J. Shi, Q. Niu, P. Tan, E. Wang, B. Liu, and J. Feng, “Valley-selective circular dichroism of monolayer molybdenum disulphide,” Nat. Commun. 3(1), 887 (2012).
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H. Zeng, J. Dai, W. Yao, D. Xiao, and X. Cui, “Valley polarization in MoS2 monolayers by optical pumping,” Nat. Nanotechnol. 7(8), 490–493 (2012).
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J. T. Ye, Y. J. Zhang, R. Akashi, M. S. Bahramy, R. Arita, and Y. Iwasa, “Superconducting dome in a gate-tuned band insulator,” Science 338, 1193 (2012).

K. F. Mak, K. He, J. Shan, and T. F. Heinz, “Control of valley polarization in monolayer MoS2 by optical helicity,” Nat. Nanotechnol. 7(8), 494–498 (2012).
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K. Kim, S. Coh, L. Z. Tan, W. Regan, J. M. Yuk, E. Chatterjee, M. F. Crommie, M. L. Cohen, S. G. Louie, and A. Zettl, “Raman Spectroscopy Study of Rotated Double-Layer Graphene: Misorientation-Angle Dependence of Electronic Structure,” Phys. Rev. Lett. 108, 246103 (2012).

2011 (1)

B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol. 6(3), 147–150 (2011).
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2010 (4)

K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010).
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K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett. 105, 136805 (2010).

A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2.,” Nano Lett. 10(4), 1271–1275 (2010).
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A. Rubano, T. Satoh, A. Kimel, A. Kirilyuk, T. Rasing, and M. Fiebig, “Influence of laser pulse shaping on the ultrafast dynamics in antiferromagnetic NiO,” Phys. Rev. B Condens. Matter Mater. Phys. 82(17), 174431 (2010).
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2009 (1)

A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene,” Rev. Mod. Phys. 81(1), 109–162 (2009).
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2008 (1)

S. Ghosh, I. Calizo, D. Teweldebrhan, E. P. Pokatilov, D. L. Nika, A. A. Balandin, W. Bao, F. Miao, and C. N. Lau, “Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits,” Appl. Phys. Lett. 92, 151911 (2008).

2005 (2)

Y. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, “Experimental observation of the quantum Hall effect and Berry’s phase in graphene,” Nature 438(7065), 201–204 (2005).
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2002 (1)

Y. G. Fokin, T. V. Murzina, O. A. Aktsipetrov, S. Soria, and G. Marowsky, “Phase transitions in ferroelectric liquid crystals probed by optical second harmonic generation,” Surf. Sci. 507-510, 724–729 (2002).
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1994 (1)

M. Fiebig, D. Fröhlich, B. B. Krichevtsov, and R. V. Pisarev, “Second harmonic generation and magnetic-dipole-electric-dipole interference in antiferromagnetic Cr2O3,” Phys. Rev. Lett. 73(15), 2127–2130 (1994).
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1991 (1)

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1989 (1)

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Aivazian, G.

S. Wu, J. S. Ross, G. Bin Liu, G. Aivazian, A. Jones, Z. Fei, W. Zhu, D. Xiao, W. Yao, D. Cobden, and X. Xu, “Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2,” Nat. Phys. 9(3), 149–153 (2013).
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Aktsipetrov, O. A.

Y. G. Fokin, T. V. Murzina, O. A. Aktsipetrov, S. Soria, and G. Marowsky, “Phase transitions in ferroelectric liquid crystals probed by optical second harmonic generation,” Surf. Sci. 507-510, 724–729 (2002).
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L. M. Malard, T. V. Alencar, A. P. M. Barboza, K. F. Mak, and A. M. De Paula, “Observation of intense second harmonic generation from Mo2 atomic crystals,” Phys. Rev. B Condens. Matter Mater. Phys. 87, 801401 (2013).

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Y. Wang, J. Xiao, H. Zhu, Y. Li, Y. Alsaid, K. Y. Fong, Y. Zhou, S. Wang, W. Shi, Y. Wang, A. Zettl, E. J. Reed, and X. Zhang, “Structural phase transition in monolayer MoTe2 driven by electrostatic doping,” Nature 550(7677), 487 (2017).

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G. Wang, X. Marie, I. Gerber, T. Amand, D. Lagarde, L. Bouet, M. Vidal, A. Balocchi, and B. Urbaszek, “Giant Enhancement of the Optical Second-Harmonic Emission of WSe2 Monolayers by Laser Excitation at Exciton Resonances,” Phys. Rev. Lett. 114, 097403 (2015).

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L. Wu, S. Patankar, T. Morimoto, N. L. Nair, E. Thewalt, A. Little, J. G. Analytis, J. E. Moore, and J. Orenstein, “Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals,” Nat. Phys. 13(4), 350–355 (2017).
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A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, O. B. Aslan, D. R. Reichman, M. S. Hybertsen, and T. F. Heinz, “Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2,” Phys. Rev. Lett. 113(7), 076802 (2014).
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S. Ghosh, I. Calizo, D. Teweldebrhan, E. P. Pokatilov, D. L. Nika, A. A. Balandin, W. Bao, F. Miao, and C. N. Lau, “Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits,” Appl. Phys. Lett. 92, 151911 (2008).

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Figures (5)

Fig. 1
Fig. 1 (a) Schematic of the generation of second harmonic generation in non-centrosymmetric materials, in which frequency of excitation is half of the generated light. (b) In 2H-MoS2, the Mo atoms are hexagonally packed within each layer and trigonal prismatically coordinated with S atoms on the top and bottom. The side projection shows the flipped orientation of each layer and the anti-parallel orientation of the SH dipoles. In (c), the absence of SHG in even layers exhibits the cancellation of SH dipoles. (d) Crystal structure of 3R-MoS2 (trilayer unit cell outlined in red). Individual layers are identical to the 2H structure, however the unit cell and bulk crystal are noncentrosymmetric. The layers are oriented in such a way that the dipoles are parallel allowing for constructive interference of SHG in (e). (b), (d), (e) are reprinted from ref [24]with permission from Springer Nature and (c) is adapted from ref [25] with permission from American Physical Society.
Fig. 2
Fig. 2 (a) Optical microscopy images for a series of MoS2 bilayers with a stacking angle distributed from θ = 0° to 60°. (b) The corresponding false color-coded SH intensity mappings of flakes shown in (a). The scale bar is 5 μm. (c) A schematic for illustrating the vector superposition of the SH electric fields, where E (ω)is the electric field of the fundamental light, E 1 (2ω)( E 2 (2ω)) is the SH electric field from the flake 1 (flake 2), and E s (2ω)is the resulting SH electric field from the stacking region. In the insets of (c), schematics for stacked bilayers with specific θ = 0°, 60° or 180°. (d) The measured angular dependence of κ(θ) for homogeneous MoS2/MoS2 bilayers. (e) Polar plot of the second-harmonic intensity from monolayer MoS2 as a function of the sample angle. And the inset shows top view of the MoS2 crystallographic orientation with respect to the incident laser polarization. Yellow and gray dots refer to S and Mo atoms respectively. (a)-(d) are adapted with permission from ref [38]. Copyright 2014 American Chemical Society. (e) is reprinted from ref [25] with permission from American Physical Society.
Fig. 3
Fig. 3 (a) Synthesis of the Janus MoSSe monolayer. Optical microscopy and atomic force microscopy images are shown for each growth step. (b) Schematics of out-of-plane induced SHG. The beam position (red) at the objective back aperture can be scanned along the x direction with a motorized stage, which tunes the incident angle accordingly. The SHG (green) is collected by the same objective and analyzed by a polarizer. (c) Angle-dependent SHG intensity ratio between p and s polarization in the Janus MoSSe and randomized alloy samples. (d) Side view of two energy-degenerate ferroelectric In2Se3 structures. Single quintuple layer consists of covalently bonded indium and selenium triangular lattices. (a)-(c) are reprinted from ref [43] with permission from Springer Nature. (d) is adapted from ref [44] with permission from American Physical Society.
Fig. 4
Fig. 4 (a) Schematics of the impact of the dimensionality on the electronic and excitonic properties, represented by optical absorption. The transition from 3D to 2D is expected to lead to an increase of both the band gap and the exciton binding energy. Reprinted from ref [50] with permission from American Physical Society. (b) Two-photon excitation map shows the strong SHG resonance. (c) SHG spectra on resonance with the exciton at selected gate biases. (d) SHG intensity maps as a function of gate voltage and emission energy. (e) Illustration of gate-dependent exciton- and trion-enhanced SHG. (b)-(e) are adapted from ref [52] with permission from Springer Nature.
Fig. 5
Fig. 5 (a) Scanning electron micrograph of the fabricated silicon photonic crystal cavity with monolayer WSe2 on top, indicated by the orange outline. Scale bar: 10μm. (b) Schematic of the device operation: two infrared (IR) photons with frequency ω (in red) resonantly couple into the photonic crystal cavity. The cavity mode interacts with the WSe2 (the transparent pink sheet), which then generates the second harmonic photon at a frequency of 2ω (in blue). (c) Cross-polarized reflectivity measurements of the higher order cavity modes before and after monolayer WSe2 transfer. The mode ‘α’ at ∼1515 nm and the mode ‘β’ at ∼1490 nm are highlighted in blue and red respectively ‘β’ at ∼1490 nm. (d) Overlay of the photoluminescence signal from WSe2 generated via a HeNe laser (black), SHG spectrum from mode α (blue), and SHG spectrum from mode β (red). (e) Schematics and measurement configuration of a MoTe2 monolayer field-effect transistor with ionic liquid gate, through which 2H phase can be converted into 1T’ phase via gating. Crystal structures of the 2H and 1T′ phases of monolayer MoTe2 are shown in the inset. (e) Typical SHG intensity modulation for this monolayer while phase changes from 2H to 1T’. (a)-(d) are reprinted from ref [53] with permission from IOP Publishing. (e)-(f) are reprinted with from ref [58] with permission from Springer Nature.

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